This invention relates to an electron beam pattern transfer system using a photoelectric transducing mask.
It has been impossible heretofore to increase the packing density of semiconductor devices, even by employing currently available fine patterning techniques (e.g., photolithography), due to the dimensional limits of a pattern resulting from the wavelength of light used. Consequently, there has been a great demand for the development of an even finer patterning technique, such as a submicron patterning technique.
A pattern transfer system utilizing an X-ray or an electron beam instead of a light beam shows promise as a new ultrafine patterning technique. According to the electron beam pattern transfer system, an ultraviolet ray is directed onto a photoelectrical transducing mask which is positioned parallel to a substrate such as a semiconductor wafer and under uniform, strong electric and magnetic fields created between the mask and the substrate a photoelectron beam pattern is focused onto a resist film on the wafer so as to transfer a mask pattern onto the resist film on the wafer. This system permits the transfer of an ultra-fine pattern onto a submicron area. The patterning technique of this kind has various advantages including, for example, the following: (1) a high-speed pattern transfer can be carried out with a high yield; and (2) since the mask is similar in structure to a photomask, the conventional technique can be used in the fabrication of the mask structure.
On the other hand, this electron beam pattern transfer system involves the following problem with respect to the wafer alignment technique. When the photoelectron beam pattern emitted from the photoelectric surface of the mask is focused onto a silicon wafer, and an acceleration voltage V is applied between the mask and the wafer, focusing magnetic field intensity B and mask-to-wafer distance d are important parameters. In order to obtain a high image resolution characteristic, it is necessary to reduce the edge blur of the photoelectron beam pattern on the wafer to a minimum. For example, in order to control the edge resolution of the electron beam pattern down to below 0.1 .mu.m, a deviation of the applied voltage V from a set value should be reduced to below 0.02% and a variation in the focusing magnetic field intensity B should be reduced to below 0.01%. It is also necessary to reduce a variation in the mask-to-wafer distance d to a very small value. However, there is a high probability that the above-mentioned distance d will vary undesirably due to, for example, the replacement of the silicon wafer (sample), the replacement of the mask, and the accuracy with which a table having the wafer placed thereon is mechanically moved. Even if the accuracy of the applied voltage V and magnetic field intensity B can be enhanced, the edge blur of the electron beam pattern is increased due to inaccurate setting of the mask-to-wafer distance d, resulting in a lowered image resolution. It is therefore difficult in the prior art technique to focus the photoelectron beam pattern onto the wafer with high accuracy by effectively adjusting the above-mentioned three important parameters: the applied voltage V, magnetic field intensity B and distance d.
In the prior art technique, for example, in order to focus an electron beam pattern onto the wafer, a pattern transfer is initially carried out and the applied voltage V and magnetic field intensity B are adjusted, while observing the transferred pattern image, to obtain an optical requirement. It is therefore impossible to adequately compensate for a variation in the mask-to-wafer distance. Furthermore, a great deal of time is required for focusing the electron beam pattern onto the wafer, thus lowering the rate of operation. It is also impossible to detect and compensate for any defocusing which may occur after focusing has been carried out. This leads to the problem of low yield in the pattern transfer operation.